The present invention relates to a transparent conductive film which is formed as a transparent electrode in contact with a semiconductor layer in a thin film transistor for use as a switching device of an active liquid crystal display device, or in a solar cell.
For instance, an active liquid crystal display device employs a thin film transistor as a switching device. Conventionally, the thin film transistor has such a structure as depicted in FIG. 1. That is, drain and source electrodes 12 and 13, respectively, each of which is formed by a transparent conductive film of so-called ITO (Indium Tin Oxide) or tin oxide, are disposed apart on a glass or like transparent substrate 11, and a semiconductor layer 14 of amorphous silicon or the like is formed between the drain and source electrodes 12 and 13. In this instance, the drain and source electrodes 12 and 13 are covered with ohmic contact layers 15 and 16 of n.sup.+ silicon so as to establish ohmic contact between the electrodes 12 and 13 and the amorphous semiconductor layer 14. The semiconductor layer 14 is covered with a gate insulating film 17 of, for example, silicon nitride, which is, in turn, covered with a gate electrode 18.
Another device which employs a transparent electrode provided in contact with a semiconductor layer is such a solar cell as shown in FIG. 2. In this case, a transparent conductive film 22 is formed on a transparent substrate 21 as of glass, and p-type, i-type, and n-type conductive layers 23, 24, and 25, respectively, of amorphous silicon are formed on the transparent electrode 22 in that order. Furthermore, an electrode 26 of aluminum or similar material is deposited on top of the n-type conductive layer 25.
In the manufacture of the device which has a transparent electrode formed in contact with a semiconductor layer, as described above, the amorphous silicon semiconductor layer 14 in the thin film transistor or the p-type amorphous silicon conductive layer 23 in the solar cell is usually deposited through a plasma assisted CVD (Chemical Vapor Deposition) process. In this case, the substrate 11 or 21 is heated to 200.degree. C. through 300.degree. C. for the formation of the semiconductor layer. The heating of the substrate causes the migration of constituent elements of the transparent conductive films 12 and 13 or 22, for example, indium and/or tin, which enters into the semiconductor layer 14 or p-type conductive layer 23 adversely affecting the fabricated devices.
In the thin film transistor, the ohmic contact layers 15 and 16 are each constituted by, for instance, an n.sup.+ -type conductive layer, and if indium is diffused into the conductive layer, it will serve as a p-type impurity, making it impossible to obtain good ohmic contact. When indium is diffused into the semiconductor layer 14, it acts as a p-type semiconductor, which impairs the operation characteristics of the thin-film-transistor. Also in the solar cell, the diffusion of indium or tin from the transparent conductive film 22 into the p-type and i-type conductive layers 23 and 24 will cut down the conversion efficiency of the cell.